The invention relates to a process and to a system for the compression of the flow rate of data transmitted between at least a television transmitter and a television receiver
In digital television, the luminance and color difference signals are digitized at 8 bits per spot, with a sampling frequency and structure dependent on the particular circumstances. This information is generally transmitted in real time and the rough flow rate is very high (specifically, higher than 140 megabits per second), which makes it necessary to reduce the flow rate so that existing transmission supports can be used.
Various processes and devices for reducing the flow rate are known. Of these, particular interest is attached to coding processes which use modulation of differential coded pulses of the television signals because of their constructional simplicity. This applies all the more in view of the fact that the transmitted data are constituted by fixed length binary code words, because of problems of managing buffer stores necessary for adapting the variable flow rate of the transmitter to the fixed flow rate of the channel connecting the transmitter to the receiver are eliminated. In addition, the use of intra-image coding makes it possible to obviate the systematic use of image stores. The known differential coding processes consist of coding the difference between the value of a sample of the signal and an estimate or prediction, calculated on the basis of the already coded preceding sample values, said difference being quantized by a quantizer with n quantization levels. With each level i is associated a code C.sub.1, which is transmitted on the transmission channel or line. The code received is converted into its real value, which is then added to a prediction value calculated by the receiver in order to restore the signal. A feedback loop makes it possible to perform at the transmitter, a prediction identical to that carried out at the receiver. According to this principle, the prediction is produced at the transmitter on the basis of the restored value of the sampled value, and at the receiver, on the basis of the restored value of the received value, which may obviously be subject to error. If there is no transmission error, the restored values and those of the predictions at the transmitter and the receiver are identical. In the opposite case, the predictions at the transmitter and receiver differ and special precautions have to be taken to avoid the error being transferred to the following samples restored by the receiver. In order to reduce the transfer of transmission errors to following samples, one solution consists of also quantizing the prediction during coding and decoding, the code allocation to the quantized prediction error being carried out during coding and decoding, as a function of the quantized prediction. A transmission error which, during decoding, leads to an error on the corresponding quantized prediction for the following value is compensated by the calculation of the prediction error, which takes account of this quantized prediction.
Embodiments of digital data differential coding--decoding devices using the aforementioned error correction method and process are described in French Patent Application No. 8,120,167, filed in the name of the present Applicant.
However, a problem arises when it is wished to apply differential coding--decoding methods to the transmission of television pictures, particularly when it is wished to obtain a significant compression of the flow rate of transmitted data, because said differential coding--decoding processes do not make it possible to drop below a 4 bit coding per transmitted picture spot, with an intra-image coding and a fixed length code for the luminance component even though the lock in definition of the image obtained at the receiver which is caused by the reduction of the quantization levels, is compensated by adding pseudo random data to the transmitted data. Below a 4 bits coding value, the restored image quality is no longer acceptable.
Thus, a high flow rate compression leads to spaced quantization levels, which causes problems of restoring the image or picture, both for the uniform image zones and for the contours of the image. In the uniform image zones, slight luminance variations are directly perceived by the eye and it would consequently by preferable to quantize the luminance signal of the uniform zones with the aid of quantizers having close levels, so as to not unduly exaggerate the limited luminance variations, which could give rise to the appearance of false contours in the vicinity of the zero prediction error. However, on the contours of the image marking the transition between two uniform zones, a quantization with the aid of spaced reconstruction levels would be preferable for better restoring the contours. However, in this second case, the spacing between two levels cannot exceed a certain limit, beyond which the contours appear restored in the form of stairs or platforms.